A survey was carried out during the late '80s over 7 infiltration percolation plants, serving populations ranging between 400 and 1700. With sand depths, hydraulic loads, influent COD and NTK concentrations respectively ranging from 0.6 to 0.2 m, 0.07 to 0.77m/day, 820 to 75 and 70 to 10 mg/l, and with different operating schedules, this set of plants displays a wide spectrum of infiltration percolation in use.

When plants are suitably designed, sized and operated, primary effluents oxidation is very effective and current EEC quality standards for wastewater treatment plant effluents are matched. Disinfection is poor, below the level expected from laboratory and pilot plant data. This is due to non-uniform spreading of the influents on the infiltration areas and exceedingly short circuits and short water detention times in the sand beds.

Based on a theoretical approach and on data obtained from these and many other plants, a sizing methodology is provided. Recommended improvements in the spreading technology, as well as in the plant design and management, should lead to more reliable oxidation and disinfection performance

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